Structural investigation of the insulator-metal transition in NiS$_{2-x}$Se$_x$ compounds

We report on a combined measurement of high-resolution x-ray diffraction on powder and Raman scattering on single crystalline NiS2-xSex samples that exhibit the insulator-metal transition with Se doping. Via x-rays, an abrupt change in the bond length between Ni and S (Se) ions was observed at the transition temperature, in sharp contrast to the almost constant bond length between chalcogen ions. Raman scattering, a complementary technique with the unique sensitivity to the vibrations of chalcogen bonds, revealed no anomalies in the phonon spectrum, consistent with the x-ray diffraction results. This indicates the important role of the interaction between Ni and S (Se) in the insulator-metal transition. The potential implication of this interpretation is discussed in terms of current theoretical models.